QCD hard scattering at J-PARC Marco Stratmann July 8 th, 2008

4th Meeting on High-Energy Hadron Physics at J-PARC2 Outline  motivation  theor. framework I: pQCD & hard scattering  expectations: pion and photon J-PARC  theor. framework II : resummations example: Drell-Yan process

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC3 I. motivation the large x frontier exploring the limits of pQCD

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC4 exploring the nucleon is of fundamental importance tool for discovery: RHIC “new state of matter” LHC Higgs, “new physics” explore and understand QCD: dynamics, models, lattice methods test our ability to use QCD: asymptotic freedom, factorization theorems, phenomenological inputs, all-order resummations know what we are made of: origin of mass, visible mass of the universe

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC5 a great place to test and learn about QCD:  models for parton densities  theor. prejudices about x ! 1 (‘‘counting rules‘‘)  exploring the limits of perturbative QCD methods  probing the transition from partonic to hadronic d.o.f. compelling reasons for hadronic J-PARC ‘‘resolution scale‘‘

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC6 g(x,Q) significant uncertainties for x & 0.3 g(x,Q) » (1-x) 0.8 ¥ 3.6 unstable w.r.t. fact. scheme, Q  latest global analysis results from CTEQ arXiv: [hep-ph] : u(x,Q) significant uncertainties for x & 0.75 best fits favor u(x,Q) » (1-x) 3 ¥ 4 ‘‘counting rules‘‘: do they count? Farrar, Jackson; Close, Sivers; Blankenbecler, Brodsky; Brodsky, Gunion; Brodsky, Schmidt;...

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC7 much less is known for helicity-dependent pdfs: 11 helicity retention -1/31 rel. const. quark model stat. parton model  d/d  u/umodel Farrar, Jackson; Brodsky, Schmidt; Brodsky, Burkardt, Schmidt; Avakian et al. Bourrely, Buccella, Soffer Isgur, Thomas, Close  conclusions so far:  d/d ! 1 a long shot... conclusions so far:  d/d ! 1 a long shot...

recent developments non-zero OAM can delay  d/d ! 1 to x ' 0.75 Avakian et al., arXiv: de Florian, Sassot, MS, Vogelsang arXiv: [hep-ph] 1 st global analysis: evidence for flavor-asymmetric pol. quark-sea awaits further exp. confirmation

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC9 the big picture: impact on LHC physics unpol. pdfs are vital for reliable predictions for new physics signals and their background cross sections at the LHC ATLAS is looking into their pdf constraining potential pdf uncertainty relevant for large Higgs masses, Htt prod.,... Djouadi, Ferrag;... high precision pdfs are crucial as they can compromise the potential for new physics discovery: high-x gluon uncertainty: reduces discovery reach in dijets 5-10 TeV ! 2-3 TeV high-x quark uncertainties: similar for Drell-Yan process Ferrag;...

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC10 II. theoretical framework (part I) pQCD & hard scattering

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC11 QCD is the theory of strong interactions – how can we make use of perturbative methods? asymptotic freedomconfinement non-perturbative hadronic structure hard scattering cross sections e.g. Lattice QCDperturbative methods goal : probe hadronic structure with weakly interacting quanta of asymptotic freedom interplay key: factorization Gross, Wilczek; Politzer

perturbative QCD can be used to make quantitative predictions if  s (p T ) << 1 (exploiting asymptotic freedom of QCD) Gross, Wilczek; Politzer key tool : factorization theorem & universality Libby, Sterman; Ellis et al.; Amati et al.; Collins et al.;... the strategy is simple, e.g. pp !  X: d  ' compute as a power series in  s in pQCD measure learn about !

… but the details are not: long-distance physics  dep. predicted by pQCD short-distance physics  calculable in pQCD as series in  s power corrections  usually safely neglected factorization requires introduction of unphysical scales  f,r information on spin/nucleon structure & hadronization is hidden inside complicated convolutions ! global analyses pQCD valid only if p T is “large enough” (  s small) – but where precisely?

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC14 the harder we work, the less the final result should depend on these artificial scales a powerful gauge of the reliability of a pQCD calculation scale dependence inherent to any pQCD calculation  there is no such thing like ‘‘the right scale‘‘ (not even Q in DIS!)  if we truncate the series after the first N terms, there will be a residual scale dependence of order N+1 ! theoretical error  a measurable cross section d(  )  has to be independent of  r and  f renormalization group eqs. like DGLAP evolution

anatomy of a NLO calculation  need loop and real emission corrections to LO diagrams  encounter singularities sing. config. of a three parton final-state ! dimensional regularization, … soft: cancel in sum collinear: absorb into def. of phenomenological inputs (parton densities & fragmentation fcts.) + a meaningful observable is insensitive to these long-dist. effects “infrared – safety”

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC16 potential problems at fixed-target energies key question: do we really talk about hard scattering if is small ?? usually NLO pQCD undershoots data example: pion producion a priori we don‘t know, but in pp collisions, hardness is controlled by the observed transverse momentum : Apanasevich et al.; Aurenche et al.; Bourrely, Soffer small !! ? not necessarily! significant improvement after resummation of large logarithms to all orders ( ! later in the talk)

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC17 III. expectations pion and photon J-PARC

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC18 potential hard probes at J-PARC energies Drell-Yan lepton pairs, inclusive pions or prompt photons, and charm NLO QCD corrections to all reactions are known: Drell-Yan prompt photons pions many papers (U,L,T) for J-PARC, see: Yokoya et al.; Kawamura et al. Aurenche et al. (U); Baer et al. (U,L); Contogouris et al. (U,L); Gordon, Vogelsang (U,L); Mukherjee, MS, Vogelsang (T) Aversa et al. (U); de Florian (U,L); Jäger et al. (U,L); Mukherjee, MS, Vogelsang (T) U: unpolarized L: long. polarized T: trans. polarized charm production will be discussed in my 2 nd talk

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC19 input to all pQCD calculations ¼ COMPASS-like angular acceptance: 200mrad 50 GeV proton beam (polarization 75%) on fixed target target polarization 75%; dilution factor 0.15; integr. luminosity: 10fb -1 choice of pdfs: CTEQ6 (U) GRSV (L) ‘‘Soffer saturated‘‘ (T) choice of frag. fcts.: KKP

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC20 single-inclusive pion production (unpolarized) scales used: p T ¥ 4p T no improvement in scale uncertainty significant NLO corrections

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC21 single-inclusive pion production (long./trans. pol.) longitudinaltransverse somewhat more moderate K-factors

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC22 single-inclusive pion production (A LL & subproc.) sensitivity to  g through qg-scattering large x probed!

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC23 single-inclusive pion production (A TT ) upper bound for A TT ‘‘Soffer inequality‘‘ saturated Soffer; Sivers as expected: A TT < A LL (lack of gluons) Jaffe, Saito proc. dominant “valence-valence” GSI-Fair

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC24 prompt photon production at RHIC we observe a strong rise but  0 is still less than one the scale dependence and K-factors are equally good/bad as for inclusive pion production ( focus on A LL & A TT ) we adopt the isolation criterion of Frixione (no fragmentation contribution; R=0.4,  =1)  /  0 ratio could be interesting to look at

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC25 prompt photon production (A LL & A TT ) A LL : well-known sensitivity to  g through qg !  q A TT : again, this is an upper bound

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC26 independent of the polarization and the process we observe a large residual scale dependence also in NLO very sizable NLO corrections interesting prospects to constrain pdfs at large x need to improve pQCD calculations beyond NLO

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC27 IV. theoretical framework (part II) resummations Drell-Yan process

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC28 resummations: why/where relevant fixed order pQCD has many successes but also failures key question: why problems in fixed-target regime and why often near perfect at colliders ?? at partonic threshold / near exclusive boundary: just enough energy to produce, e.g., high-p T parton ‘‘inhibited‘‘ radiation (general phenomenon for gauge theories) simple example: Drell-Yan process “imbalance” of real and virtual contributions: IR cancellation leaves larges log’s

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC29 logarithms related to partonic threshold for single-inclusive pp cross sections: general structure of partonic cross sections at the k th order: “threshold logarithms” where relevant? … convolution with steeply falling parton luminosity z = 1 emphasized, in particular as  ! 1

resummation of such terms has reached a high level of sophistication Sterman; Catani, Trentadue; Laenen, Oderda, Sterman; Catani et al.; Sterman, Vogelsang; Kidonakis, Owens;... resummations: how it is done may spoil perturbative series - unless taken into account to all orders worked out for most processes of interest at least to NLL well defined class of higher-order corrections often of much phenomenological relevance even for high mass particle production at the LHC resummation (= exponentiation !!) occurs when “right” moments are taken: fixed order calculations needed to determine “coefficients” the more orders are known, the more subleading logs can be resummed Mellin moments for threshold logs

31 Fixed order Resummation LLNLLNNLL N LO k NNLO NLO · · · · · · · · · · · · LO resummations: general structure (slide from a talk by W. Vogelsang)

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC32 resummations: some LL exponents DIS prompt photons inclusive hadrons moderate enhancement, unless x Bj large expect much larger enhancement exponents positive enhancement unobserved parton Sudakov ‘‘suppression ‘‘ observed partonsunobserved e.g. (assuming fixed  s )

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC33 resummations: some phenomenology de Florian, Vogelsang soft-gluon K-factors relative to NLO very large enhancement in NLL in good agreement with data much reduced scale dependence

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC34 resummations: window to the non-pert. regime important technical issue: resummations are sensitive to strong coupling regime ! need some prescription to avoid Landau pole Catani, Mangano, Nason, Trentadue: define resummed result such that series is asymptotic w/o factorial growth associated with power corrections ! power corrections/k T effects may be added afterwards if phenomenologically needed studying power corrections prior to resummations makes no sense window to the non-perturbative regime so far little explored

resummations: J-PARC almost no quantitative studies at J-PARC energies yet; expect: very significant effects on cross sections partial cancellation of soft-gluon effects in A LL in particular for prompt photons (simple color structure) reduction for A TT (lack of gluons in d  ) resummations seem to be mandatory at fixed-target energies technical framework available well-defined & systematic improvement of pQCD results often much reduced scale uncertainties window to non-perturbative regime notable exception: Drell-Yan process (of much relevance for J-PARC programme) d  /dM, A LL, A TT Yokoya; Shimizu, Sterman, Vogelsang, Yokoya d  /dM/dq T, A TT Kawamura, Kodaira, Shimizu, Tanaka

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC36 Drell-Yan lepton pairs at J-PARC fixed order results are known up to NNLO Hamberg, van Neerven, Matsuura; Harlander, Kilgore; Anastasiou, Dixon, Melnikov, Petriello (from H. Yokoya)  NNLO corrections still fairly significant  large corrections stem from near partonic threshold z ' 1

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC37 Drell-Yan process: threshold resummations first three towers of logs known: LL, NLL, NNLL (from H. Yokoya) nice convergence NNLL sufficient expansion reproduces f.o. results ! f.o. dominated by threshold logs

July 8th, 20084th Meeting on High-Energy Hadron Physics at J-PARC38 V. concluding remarks scientific J-PARC

ground breaking ceremony in , almost ready to go many avenues for important measurements exploring the limits of pQCD